U.S. patent application number 11/363560 was filed with the patent office on 2006-08-31 for method for producing high quality optical parts by casting.
Invention is credited to Eugene Giller, Noa M. Rensing, Paul M. Zavracky.
Application Number | 20060192307 11/363560 |
Document ID | / |
Family ID | 36928072 |
Filed Date | 2006-08-31 |
United States Patent
Application |
20060192307 |
Kind Code |
A1 |
Giller; Eugene ; et
al. |
August 31, 2006 |
Method for producing high quality optical parts by casting
Abstract
A casting method, rather than injection molding, to produce
polymer optical components and systems is provided. The casting
process controls shrinkage and stress, thus providing both high
bulk uniformity and high quality, accurate surfaces, by
incorporating polymer films into the mold. The films may remain
incorporated into the part or may optionally be removed from the
part after removal from the mold. In addition, the incorporation of
separately produced components within the cast part is also
provided, eliminating post-casting assembly manufacturing
steps.
Inventors: |
Giller; Eugene; (Needham,
MA) ; Rensing; Noa M.; (Newton, MA) ;
Zavracky; Paul M.; (Norwood, MA) |
Correspondence
Address: |
WEINGARTEN, SCHURGIN, GAGNEBIN & LEBOVICI LLP
TEN POST OFFICE SQUARE
BOSTON
MA
02109
US
|
Family ID: |
36928072 |
Appl. No.: |
11/363560 |
Filed: |
February 27, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60656219 |
Feb 25, 2005 |
|
|
|
Current U.S.
Class: |
264/1.7 |
Current CPC
Class: |
B29D 11/00009
20130101 |
Class at
Publication: |
264/001.7 |
International
Class: |
B29D 11/00 20060101
B29D011/00 |
Claims
1. A method of producing a solid optical system having a protruding
optical element, the element providing optical power, comprising:
providing a mold assembly having a mold cavity, the mold assembly
including a recess in one surface shaped to receive the protruding
optical element; introducing an optical material into the mold
cavity, at least a portion of the optical material comprising an
optical polymerizable casting compound and a further portion of the
optical material disposed within the recess to form the protruding
optical element; and curing the polymerizable casting compound to
provide an optical component; and removing the optical component
from the mold assembly.
2. The method of claim 1, wherein the further portion of the
optical material forming the protruding optical element comprises a
lens formed prior to the step of intruding the optical material
into the mold cavity.
3. The method of claim 2, wherein the protruding optical element is
held in the recess by vacuum, gravity, or a temporary adhesive.
4. The method of claim 2, wherein an outer surface of the
protruding optical element is protected with a layer of a
protective material.
5. The method of claim 4, wherein the protective material is
applied as tape.
6. The method of claim 4, wherein the layer of protective material
is applied as a liquid.
7. The method of claim 4, wherein the layer of protective material
is applied as a vapor.
8. The method of claim 1, wherein the mold assembly comprises faces
comprised of polished polycarbonate plates.
9. The method of claim 8, wherein the polished polycarbonate plates
comprise opposed surfaces.
10. The method of claim 1, wherein at least a portion of the mold
assembly comprises optically polished flat surfaces.
11. The method of claim 1, wherein the mold assembly comprises a
lower element, an upper element, and a spacer element between the
lower element and the upper element.
12. The method of claim 1, wherein the spacer element includes a
surface shaped to produce an input face into a light pipe and a
further surface shaped to produce a fold mirror surface at an end
of the light pipe opposite the input face.
13. The method of claim 1, wherein the recess is polished with a
surface finish.
14. The method of claim 1, wherein the recess is formed in the mold
assembly by forming a sheet of polycarbonate against a surface
having a positive shape.
15. The method of claim 14, wherein the sheet of polycarbonate is
formed against the positive shape by heating the polycarbonate.
16. The method of claim 14, wherein the sheet of polycarbonate is
formed against the positive shape by pressing the
polycarbonate.
17. The method of claim 14, wherein the sheet of polycarbonate is
formed against the positive shape by vacuum forming.
18. The method of claim 14, wherein the positive shape is formed in
a metal or glass surface.
19. The method of claim 1, wherein the mold assembly is formed of a
polymer compression molded from a positive metal form having a
shape corresponding to a desired finished part.
20. The method of claim 1, wherein the mold assembly comprises a
injection molded or cast polycarbonate.
21. The method of claim 1, wherein at least a portion of the cavity
in the mold assembly is lined with a film material.
22. The method of claim 21, wherein the film is shaped to the mold
cavity prior to introduction of the casting compound into the mold
cavity.
23. The method of claim 21, wherein the film is shaped to the mold
cavity during introduction of the casting compound into the mold
cavity.
24. The method of claim 21, wherein the film is removed from the
optical component after the optical component is removed from the
mold.
25. The method of claim 21, wherein the film remains in the mold
when the optical component is removed from the mold.
26. The method of claim 21, wherein the casting compound is
selected to optimize bulk mechanical and optical properties of the
optical component, and the film is selected to optimize mechanical
and optical properties of the surface of the optical component.
27. The method of claim 26, wherein the casting material is
optimized for low stress and high optical uniformity.
28. The method of claim 21, wherein the casting compound comprises
a solid when cured.
29. The method of claim 21, wherein the casting comprises a gel or
a liquid when cured and the film is sufficiently rigid to retain
the gel or the liquid.
30. The method of claim 21, wherein after curing the casting
compound is softer than the film.
31. The method of claim 21, wherein the film has an index of
refraction equal to or lower than an index of refraction of the
casting compound when cured.
32. The method of claim 21, wherein a coupling agent is provided
between the film and the casting compound.
33. The method of claim 21, wherein the film is optically
clear.
34. The method of claim 21, wherein the film provides abrasion
resistance.
35. The method of claim 21, wherein the film provides moisture
resistance.
36. The method of claim 21, wherein the film provides resistance to
chemical attack.
37. The method of claim 21, wherein the film provides an
anti-reflection coating.
38. The method of claim 21, wherein the film provides a hard
coating.
39. The method of claim 21, wherein the film provides an
anti-smudge coating.
40. The method of claim 21, wherein the film provides
polarization-dependent properties.
41. The method of claim 21, wherein a pressure difference is
created between the film and a mold surface, wherein the film
conforms to the mold surface.
42. The method of claim 21, wherein the mold assembly at least
adjacent the mold surface is porous.
43. The method of claim 42, wherein the film is shaped to the mold
surface with air or gas pressure.
44. The method of claim 21, wherein the film is shaped to the mold
surface with a complementary shaped tool.
45. The method of claim 21, wherein the film comprises a pouch
having an opening therein, the film is placed in the mold cavity,
and the casting compound is introduced into the pouch.
46. The method of claim 21, wherein the film comprises a pouch
filled with the casting compound, the pouch is placed in the mold
cavity, and shaped within the mold cavity.
47. The method of claim 21, wherein the film is formed to a mold
surface by vacuum forming.
48. The method of claim 21, wherein the film is formed to a mold
surface by blow molding.
49. The method of claim 21, wherein the film is formed to a mold
surface by heat fitting with an insert conforming to the mold
surface.
50. The method of claim 21, wherein the film is formed by vapor
phase deposition.
51. The method of claim 21, wherein the film is formed by dip
coating.
52. The method of claim 21, wherein the film is formed by spin
coating.
53. The method of claim 21, wherein the film is applied as a liquid
having a surface tension that compensates for surface roughness of
a mold surface of the mold assembly.
54. The method of claim 21, wherein the film remains on the optical
component after the optical component is removed from the mold
assembly.
55. The method of claim 21, wherein the film remains in the mold
assembly after the optical component is removed from the mold
assembly.
56. The method of claim 21, wherein the film is removed from the
optical component after the optical component is removed from the
mold assembly.
57. The method of claim 21, wherein the film is smoother than the
surface of the mold assembly.
58. The method of claim 1, wherein the mold assembly is comprises
of glass, acrylic, steel, or nickel plated steel.
59. The method of claim 1, wherein the mold assembly is made by a
rapid prototyping method.
60. The method of claim 1, wherein the mold assembly is comprised
of a sintered metal.
61. The method of claim 1, wherein the mold assembly is comprised
of a polycarbonate.
62. The method of claim 61, wherein the mold assembly is
disposable.
63. The method of claim 61, wherein the mold assembly is reusable
for a limited number of cycles.
64. An optical component having uniform bulk optical properties and
highly polished optical surfaces and incorporating a protruding
optical element formed by the method of claim 1.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit under 35 U.S.C. .sctn.
119(e) of U.S. Provisional Patent Application No. 60/656,219, filed
on Feb. 25, 2005, the disclosure of which is incorporated by
reference herein.
[0002] This application is related to U.S. patent application Ser.
No. 11/065,847, filed on Feb. 25, 2005, the disclosure of which is
incorporated by reference herein.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0003] N/A
BACKGROUND OF THE INVENTION
[0004] In the last century, polymers have increasingly been used to
fabricate low cost, low weight optics, especially in high volume
applications that also have less stringent optical requirements. An
additional advantage of plastic elements over the more traditional
glass is the ease of fabricating non-spherical (aspheric) surfaces,
which can simplify the optical design, reduce the number of
required elements, or improve the system performance. The use of
plastic in more demanding applications, however, has been limited
primarily because its optical performance does not yet match that
of glass because of poorer surface quality, inhomogeneous bulk
properties, or both. A typical problem with molded plastic optics
is birefringence due to stress induced in the molding process. One
alternative is to combine glass and plastic in the same optical
system to take advantage of the properties of glass for high
performance components. This has been done in some applications,
but designs requiring bonding glass to plastic can be difficult to
fabricate, and may not reduce the weight sufficiently. It is thus
necessary to develop new approaches to the fabrication of polymer
optical parts that maintain the advantages of plastic but match the
performance of glass.
[0005] One common approach to the fabrication of low cost optics is
to form acrylic by injection molding. Acrylic polymer is easy to
mold and gives relatively good surface quality. The molding
process, however, produces flow lines and stress gradients that
result in birefringence and optical index inhomogeneities that
distort the image. This effect is a commonly recognized problem for
more highly birefringent materials such as polycarbonates, but can
be seen in acrylic as well, especially in large or more complex
parts or in highly demanding optical systems. The birefringence of
acrylic is lower than that of other plastics, although still higher
than that of most optical glass. The distortion reduces the
resolution of the system below that of the equivalent glass
components.
[0006] Consider for example an optical system for an eyeglass
clip-on display as in FIG. 1, consisting of a compact backlight
illuminating a flat panel microdisplay, for example an LCD, and a
simple magnifier optical system with long path length. FIG. 2 shows
a simple case for the optical system consisting of a solid prism,
which may be polymer or glass, with a fold mirror, most commonly at
45.degree., and a curved surface providing optical power, most
simply a plano-convex lens with focal length comparable to but
slightly longer than the length of the prism. Optically, this
system uses a simple magnifier to create a large virtual image of
the display at a comfortable viewing distance and position. Other
more complicated optical systems may be also be used. The light
path passes laterally through the prism (or eyeglass lens, in the
case of an embedded system), entering near the eyeglass temple,
traveling through the material, and finally folding toward the
viewer in front of the eye. The primary optical power element is
placed near the eye, but the system may incorporate field lenses or
other auxiliary optical elements as well. This approach places
stringent requirements on the optical uniformity of the
polymer.
[0007] To understand these requirements, consider that a typical
pixel size for the display is 12 .mu.m, and the path length through
the polymer is 20-30 mm depending on the system's optical design
details. Thus the path length through the polymer part is long
relative to the requisite resolution. Any variations in the index
of refraction among the different possible optical paths within the
prism, for example, due to stress in the part, result in
differences in the effective optical path length of the paths. This
then degrades the focus of the system. The high temperatures and
pressures of injection molding technologies are particularly prone
to produce stress and flow lies at the corners, edges, and surfaces
of the parts. Attempts at standard injection molding by several
commercial injection molders have failed to achieve the requisite
uniformity in index of refraction. While it is possible to improve
the uniformity and control stress by changing the molding
conditions, optimizing the bulk properties can, however, result in
additional shrinkage at the mold interface, leading to inaccurate
surfaces. In the case of clip-on eyeglass displays, deviations from
flatness in the flat parallel sides of the light pipe degrade the
quality of the view of the external scene through the pipe,
resulting in an effective occlusion in the wearer's peripheral
vision.
[0008] Using techniques known in the art, it is possible to
fabricate a polymer optical system with the necessary high
resolution. FIG. 3 shows such a system comprising a solid prism or
light pipe with an input surface, a 45.degree. surface supplied
with a reflective coating, and an output surface at 90.degree. to
the input surface and facing the eye of the user. The output
surface and the back surface of the pipe parallel to the output
surface and offset from it are made transparent and highly flat to
allow the user to see through the light pipe with an unimpeded
view. The other two surfaces of the pipe, parallel to each other
and at 90.degree. to both the input and output surfaces may be
optionally transparent, diffuse, or opaque. The prism or light pipe
can be mechanically fabricated from larger polymer pieces, which
may be produced by a number of methods known to produce highly
homogeneous, low stress material. Injection molding may be used to
produce the larger polymer blank, since the fabrication process
removes the highly stressed or otherwise flawed material near the
molded surfaces and edges. Since the prism is mechanically
fabricated and either mechanically or chemically polished, the
shape and surface quality of the larger part do not matter. The
optical power is provided by a separate optical lens element
comprising a curved surface that refracts the light emitted from
the display and an opposing flat side that is cemented or otherwise
attached to the output surface of the prism adjacent to the fold
mirror in a separate fabrication step. This lens element may be
produced by injection molding or another process known in the art.
The optical path through the lens material is short and therefore
bulk inhomogeneities are less critical, and the established
processes are capable of producing accurate surfaces with
acceptable quality.
[0009] However, processes currently known in the art cannot
simultaneously produce an optical part with a uniform bulk index of
refraction free of birefringence, a 45.degree. fold surface, and
geometrically accurate, high quality optical surfaces with good
mechanical properties. The incorporation of the aspheric lens in
the fabrication of what otherwise was a flat component (eyeglass
lens or pipe) tightens the constraints on the quality of the flat
surface as well. The addition of a protruding lens makes polishing
and/or lamination difficult, and introduces a requirement that the
surface meet its flatness requirement without post-processing.
SUMMARY OF THE INVENTION
[0010] A further promise of molded plastic optics, however, is the
ability to manufacture complicated shapes with protrusions or
surface discontinuities. It is desirable to develop a process
capable of casting a monolithic system, consisting of a structure
with generally parallel faces except for a protrusion that forms
the magnifying lens surface. The monolithic process would present
an improvement in quality and cost.
[0011] This invention relates to methods for fabricating optical
systems either from cast polymers or from cast polymers with
embedded elements. It addresses the requirement for producing
complex shaped parts that have simultaneously low internal stress
and high quality optical surfaces.
[0012] The present invention addresses the need to achieve good
bulk homogeneity while maintaining high surface qualities such as
accurate geometry, mechanical hardness, and optical polish. The
present invention addresses the complication of fabricating parts
with protrusions, as well as the need to decouple the bulk index
uniformity from the surface quality of the finished part.
[0013] The present invention pertains to methods for producing
prisms and complete optical systems by casting polymers. The
casting process typically uses lower pressures and temperatures
than injection molding, resulting in lower stress and shrinkage
than injection molding. The methods described herein produce parts
with highly uniform bulk optical properties as well as highly
polished flat or curved surfaces and/or protrusions as required by
the optical design. In addition the parts may incorporate other
optical elements previously fabricated by casting, cutting,
molding, or other methods. The methods described herein offer an
innovative approach to achieving parts with a highly uniform bulk
index of refraction as well as highly polished geometrically
accurate surfaces, which may optionally include protruding
elements. This method can be used to manufacture clip-on light
pipes or embedded optical systems such as described in U.S. Pat.
Nos. 6,023,372 and 5,886,822. Here we describe modifications to the
casting approach that allow the casting of more complicated shapes
while at the same time reducing or eliminating expensive post
processing or assembly steps.
DESCRIPTION OF THE DRAWINGS
[0014] The invention will be more fully understood from the
following detailed description taken in conjunction with the
accompanying drawings, in which:
[0015] FIG. 1 illustrates a prior art optical engine for clip-on
eyeglass display, comprising backlight, LCD display, light pipe,
fold mirror, and cemented lens;
[0016] FIG. 2 illustrates a prior art light pipe and lens
assembly;
[0017] FIG. 3 is an exploded view of a prior art light pipe
assembly fabricated from an optical light pipe, a separately molded
lens, and optical cement;
[0018] FIG. 4 illustrates an exploded view of upper and lower mold
parts and lens insert according to the present invention;
[0019] FIG. 5A illustrates a casting mold for a light pipe with
parallel top and bottom optical surfaces according to the present
invention;
[0020] FIG. 5B illustrates a casting mold for monolithic
fabrication of light pipe assembly, with corresponding cavity for
an optical element in an upper mold part according to the present
invention;
[0021] FIG. 6 illustrates a method of forming a cavity in an upper
mold part by applying heat and pressure to form it against a
positive shaping element according to the present invention;
[0022] FIG. 7A illustrates a casting mold lined with barrier film
according to the present invention;
[0023] FIG. 7B illustrates a cast part with films, which may
optionally peel off the finished cast part;
[0024] FIG. 8 illustrates a prepared mold with an inserted film not
yet shaped to conform to the mold;
[0025] FIG. 9 illustrates a prepared mold with a film shaped to
conform to the mold prior to the introduction of casting resin;
[0026] FIG. 10 illustrates film pouches to be inserted into the
mold prior to the introduction of casting resin according to the
present invention; and
[0027] FIG. 11 illustrates fabrication of a cast part utilizing
pouches previously filled with casting resin and sealed prior to
the insertion of the pouch into the mold according to the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0028] In the present invention, the difficulties of creating
optical systems with protruding lenses on otherwise flat surfaces
are overcome. Three method embodiments are presented. The first
embodiment has optical elements including lenses, mirrors, etc.
incorporated into the mold prior to filling. The second embodiment
creates a low use mold from a non-machinable molding plate. The
third embodiment incorporates films into the mold. Each embodiment
is detailed below.
[0029] In a first embodiment of a method according to the present
invention, optical elements are anchored in a mold. Thus, the mold
is designed to hold one or more external optical elements such as
mirrors or previously produced lenses. The external optical
elements may be injection molded, polished, or produced by any
other manner known in the art, prior to their placement in the mold
and the introduction of a casting medium. The external optical
element may, for example, be an injection molded acrylic lens,
which may be placed in a recess within the mold, the recess being
shaped to fit a portion of the external element. The purpose of
incorporating the elements into the casting mold is to attach them
to the external surfaces of the cast part while avoiding the need
to cement them at a later step. This may result in less expensive,
more accurate, and more durable optical systems.
[0030] FIG. 4 illustrates a mold assembly 10 having a mold cavity
12 defined by a bottom element or plate 14, a spacer element 16
shaped to provide the input surface 18 and the 45.degree. fold
surface 20, and an upper element 22 that shapes the output surface
24 of the light pipe. The upper element contains a recess or
indentation 26 shaped to accept a previously fabricated external
element 28 to provide optical power, for example a lens. Upon
polymerization, a casting medium in the cavity defined by the
bottom, spacer, and upper elements permanently adheres to the
exposed surface of the optical element.
[0031] The external optical element 28 may be held in place in the
mold using a variety of methods including but not limited to
gravity, vacuum, or temporary (removable) adhesive. The attachment
method is designed to preclude the flow of the casting medium to
the external side of the lens, for example due to wicking or
vacuum. Alternatively, the sensitive surface of the external
element may be protected, for example with tape or other protective
layer, which may, for example, be applied as a liquid or vapor and
allowed to solidify either prior to the introduction into the mold
or after placement in the mold. The casting medium would then
adhere only to the exposed surfaces of the lens upon solidification
in the mold. A protective film may be kapton tape or other
protective adhesive tape known in the art. Protective coatings may,
for example, be Teflon-based coatings such as are commercially used
for the coating of ophthalmic lenses, which may be vapor deposited
or dip coated. Other vapor deposited and/or liquid coating
formulations with low adhesion to the liquid polymer medium are
also known in the art. These coatings prevent the casting medium
from adhering to and permanently contaminating the external optical
component. In the case of liquid or vapor deposited protective
coatings, it is important that the side of the part adjacent to the
casting medium be kept clean of the coating to prevent interfering
with the bond to the cast portion of the system.
[0032] In a further embodiment, alternative mold materials are
used. A casting process of the present inventors described in U.S.
patent application Ser. No. 11/065,847, filed on Feb. 25, 2005, for
fabricating flat parallel faces utilizes polished polycarbonate
plates for the faces of the mold. This is convenient because
polycarbonate sheets with a suitable finish are readily
commercially available and release easily from the cast polymer,
often without the need for additional mold release agents. Also,
the process of casting against polycarbonate does not require the
use of temperatures that are higher than the Tg (glass transition
temperature) of most polymers.
[0033] FIG. 5A shows a mold assembly 30 for producing flat faced
light pipes. The mold has a lower element 32, a spacer element 34
shaped to produce the input and fold mirror surfaces 36, 38 of the
light pipe, and an upper element 40. The upper and lower mold
elements have optically polished flat surfaces 42. The polished
surfaces are replicated in the finished part to produce the output
surface and opposing surface of the light pipe. To add a protrusion
to the optical part, a corresponding recess or indentation 44 is
formed in the mold assembly, as shown in FIG. 5B, using reference
numerals corresponding with FIG. 5A for like parts. The mold has a
lower element 32 with a polished flat surface 42, a spacer shaped
34 to produce the input and fold mirror surfaces 36, 38 of the
light pipe, and an upper element 40. The upper element has a
polished flat surface 42 corresponding to the see-through portion
of the light pipe and a shaped recess or indentation 44
corresponding to the optical power element of the light pipe
assembly. Ideally, this indentation would have the surface finish
desired in the finished part. This is a challenge in a
polycarbonate mold element because cutting, including
diamond-turning, usually results in poor surface finish and
polycarbonate is difficult to polish. Thus, an alternative to
cutting the indentation is to form the mold part 40 against a
positive 50 of the desired shape, as in FIG. 6. This entails, for
example, forming a sheet of polycarbonate against a metal or glass
surface 52 of the desired positive shape by one or a combination of
methods which may include but are not limited to heating, pressing,
or vacuum forming. The positive element 50 can be made of a
material that is easy to polish. It is also possible to create a
positive metal form of the final desired shape and then compression
mold a polycarbonate or other polymer mold from it. Another
approach involves injection molding or casting the polycarbonate
mold. The life of the shaped polycarbonate mold part is generally
shorter than that of traditional molds, however. Thus, a new mold
part must be produced regularly to replace the worn mold part.
[0034] In a further embodiment of the present invention, a film is
incorporated into the molding process. Many preferred casting
materials for optical components have low shrinkage and good
adhesion, but because of this do not release easily from many of
the materials that may be chosen for fabricating molds, such as
glass, acrylic, steel, nickel plated steel and others. Accordingly,
the present invention expands the mold options by decoupling the
surface properties from the bulk properties of the part.
[0035] One method is by casting with a barrier film between the
cast material and the mold. FIG. 7A shows a mold assembly 60 lined
with a film 62 on one molding surface 64. The film may be shaped to
the mold either prior to the introduction of the casting medium or
during the molding process. As it starts out and remains in the
solid state, it does not adhere to the mold but remains with the
part after unmolding the part from the mold. If desired, the film
can be peeled off the part after unmolding. FIG. 7B shows films 62
adhered to two surfaces of the fabricated and unmolded part 66 and
being peeled from one surface. In general, this process allows the
fabrication of parts with fewer compromises between the optical or
mechanical properties of the bulk material and the surface
properties. Integrating the film into the finished part may have
other desirable effects. The mechanical and optical properties of
the surface can be provided by the film while the cast material can
be engineered to optimize the bulk properties. Specifically, the
bulk material can be optimized for low stress and high optical
uniformity, even if the material cannot form a suitable surface,
either because it is too soft for the intended application, or if
it won't adequately replicate the mold surface. If the films are
sufficiently rigid, the bulk material may be a gel or even a
liquid.
[0036] The film or membrane used for this effort must be robust
enough to survive the handling required to place it into the mold
without folds, tears, or wrinkles, and flexible enough to stretch
to completely conform to the desired shape. Upon demolding, it must
not induce stress in the bulk material. In addition any tendency to
flow over time, induce flow in the bulk, or delaminate from the
bulk must be avoided. In the preferred method, the film is
stretched to conform to the mold. This stretching may introduce
strain and birefringence, but its impact on the overall optical
resolution of the part is minimal since the film thickness, and
therefore the optical path through the film, is short.
[0037] The film must be optically clear. It is further desirable
that the index of refraction of the film either match that of the
bulk material, or, preferably be slightly lower to provide
antireflection properties.
[0038] If the film is to remain integrated into the finished part,
the interface between the film and the cast compound must be
optically clear and resistant to delamination. It is possible to
pretreat the films with coupling agents to enhance adhesion and
reduce delamination. Some coupling agents include:
[0039] 3-aminopropylmethyldiethoxysilane,
[0040] 3-glycidyloxypropyltrimethoxysilane,
[0041] vinyltriethoxysilane,
[0042] 3-mercaptopropyltrimethoxysilane,
[0043] 3-isocyanatopropyltriethoxysilane,
[0044] triphenyl borate,
[0045] trimethoxyboroxine,
[0046] tetracresyl titanate,
[0047] tetra-2-ethylhexyl titanate,
[0048] zirconium tetra-2-ethylhexanoate,
[0049] tetraphenoxy zirconate, and
[0050] tetra-2-ethylhexylzirconate.
[0051] The material used in the films may have significantly better
mechanical and chemical properties than would otherwise be
achievable. These properties may include abrasion resistance and
resistance to moisture and chemical attacks. This is desirable as
it allows the optics to be used in environments and applications
that may otherwise be too aggressive. In addition the film may have
an antireflective coating, hardcoating, anti-smudge coating and/or
polarization dependent properties. For example, coatings are
commonly available on substrate films including polycarbonates such
as Lexan film, cast and extruded polyurethane films, and
fluorinated polymer films such as Teflon AF11 and Cytop12. To be
effective the coating must not fracture from the stresses
introduced in the molding process; this may require the
modification of standard coating processes, such as by using
thinner layers or lower temperatures.
[0052] The barrier film 72 may be anchored in or to the mold
assembly 70 prior to introducing the liquid optical polymer, as
shown in FIG. 8. A liquid polymer casting compound would then be
introduced under a pressure differential, stretching the film into
the lens recess 74 and completely filling the mold. A suitable
opening (not shown) may be formed in the mold assembly 70, for
example, through the spacer element 76, to allow the introduction
of the casting compound. Various methods of inserting, attaching,
or anchoring the film or membrane in the mold are possible. One
approach is to clamp the film in the mold. Subsequently, a pressure
difference is generated between the film and the mold so that the
film completely conforms to the mold. The upper mold element 78 may
be made porous in order to facilitate generating a pressure
difference. The pores or other openings used to generate the
pressure difference must be shaped in a suitable way to avoid
leaving impressions on the critical surfaces of the optical part.
The anchoring method must prevent any casting material from seeping
between the film and the mold, as that would deform the part and
may permanently damage the mold. Alternatively, the film 72 could
be shaped to the mold assembly 70 prior to filling it, for example
by air or gas pressure or with a complementary shaped tool. See
FIG. 9. Another approach shown in FIG. 10 is to create a pouch 82
from the film membrane. The pouch is sealed except for an opening
84 through which the casting resin is introduced after the pouch is
inserted into the mold assembly 80. The pouch can be made to
conform to the mold using a pressurized gas forming process such as
blow molding. The gas can be introduced through the opening
provided for the casting resin or through a separately provided
opening. Alternatively, the pouch can be expanded during the
process of filling with the liquid polymer resin. The pressure of
the casting compound then causes the film to conform to the mold in
a manner similar to blow molding. In yet another alternative, shown
in FIG. 11, sealed pouches 92 can be pre-filled with the casting
resin 94 and inserted into the mold assembly 90 for final shaping
and curing, similar to compression molding. An advantage of this
approach is that the casting resin can be handled in a very clean
environment, reducing the risk of contamination or inclusions. For
this approach the casting resin must be chosen so that
polymerization can be initiated at the appropriate time, for
example by the use of UV radiation or thermal energy.
[0053] The films can also be preformed to the mold surface by the
vacuum forming, blow-molding, or heat fitted with an additional
insert to conform the mold surface. An alternative to solid films
is to form a film on the mold by vapor phase deposition, dipping or
spin coating. Since this coating will be thin, its composition can
be chosen to optimize the surface quality with reduced concern as
to its bulk optical properties. Furthermore, the stress in this
layer will not be as great as in a more highly constrained
system.
[0054] The film should conform to the mold sufficiently to
replicate the desired surface features, including for example the
aspheric shape of the magnifying optic discussed above, and a small
radius inside corner at the junction between the lens and the flat
surface of the light pipe. However, it may not be desirable to
reproduce the roughness of the surface of the mold. The tension in
a solid film can provide a planarization effect leading to smoother
optical surfaces. Alternatively, if a rough mold were to be coated
with a liquid film that does not remain with the cast part
(adhering to the mold or peeling off) the surface tension of the
free surface of the liquid coating could provide the necessary
planarization. This permits the mold to be made to a coarser, and
consequently less expensive, surface finish specification. For
example, it may be possible to machine a steel mold using ordinary
CNC machine tools rather than diamond turning, or it may be
possible to forgo one or more polishing steps in the preparation of
the mold. If the smoothing effect of the film is sufficient, it may
also be possible to mold optical quality parts using a mold made by
a rapid prototyping method such as SLA. Furthermore, the
planarization effect of the film may allow a porous mold component
to be used, for example for the purpose of vacuum forming the
films. The component may be made porous by drilling or otherwise
cutting ducts into the components, or by the use of a porous
material such as sintered metal, or by other means known in the
art. The tradeoff between accurate reproduction of small surface
details and a desirable surface smoothing effect leads to a
specification on the formability of the film, which may be
different for different parts.
[0055] The desired film properties may best be defined using known
methods of finite element computer modeling or other numerical
calculations, as would be known in the art. These calculations use
the film thickness, pressure differential across the film, maximum
size of pore or other imperfection in the mold surface, and
mechanical tension in the film, along with mechanical properties
such as compliance to derive the maximum deviation from the
prescribed surface geometry. The maximum allowable surface
deviation (irregularity) may be calculated using optical modeling
computer programs such as ZEMAX, OSLO, Code V, or any other
suitable software. This information may then be used to optimize
the selection of film for lining the mold cavity.
* * * * *